1. Field of the Invention
The present invention relates to a filter device used in a system for resist-treating substrates, such as semiconductor wafers.
2. Description of the Related Art
In a photoresist-treatment process for the manufacture of a semiconductor device, a resist is applied to the surface of a thin film of, e.g., SiO.sub.2 formed on a semiconductor wafer, and the resulting resist film is exposed for a predetermined pattern and developed, thus forming a mask pattern. A resist-treatment system is used to carry out this process.
The resist-treatment system comprises an adhesion treatment unit, resist coating unit, exposure unit, developing unit, etc. A processing section (including all these units except the exposure unit) of the system is provided with a transportation path, and the resist coating unit and the baking unit are arranged on either side of the path. A transportation arm mechanism, which can travel on the transportation path, serves to transport the wafer to the adhesion treatment unit and the resist coating unit. The exposure unit is connected as an external mechanism to the processing section.
Photolithography has recently been improved with the rapid progress of highly integrated devices. For example, a 4-megabit DRAM, among other DRAMs, requires a pattern line width of about 0.7 to 0.8 .mu.m. In this case, g- or i-rays are used as an exposure light source, and a novolak-based resin is used as a resist material.
However, devices of the next generation, e.g., DRAMs with a capacity of 16 or 64 megabits or more, require reduction of the source light wavelength, and ultraviolet rays, such as excimer laser beams, are expected to be used as a promising light source. Since the novolak-based resin has a considerably high absorption factor for short-wavelength light, however, the perpendicularity (shape) of an etched resist side wall is poor.
Recently, therefore, a resist material of the chemical-sensitization type has started to be used in place of the novolak-based resin. When the material of this type is exposed, an acid is generated from its sensitizer. This acid is diffused by baking, and acts as a catalyst. By the agency of the acid, a base resin, as a main component of the resist material, is decomposed or changed in molecular structure. As a result, the resist material becomes soluble or insoluble in a developing agent.
The following is a description of features of various resist materials of the chemical-sensitization type.
(1) Hydroxyl groups in a polymer, such as polyvinyl phenol, are blocked with alkyl groups and silyl groups, and this blocking is removed by means of an acid to restore the alkali solubility (positive resist).
(2) A substance with a dissolution preventing effect is mixed with the novolak resin, and a dissolution preventing component is decomposed and denaturalized to restore or accelerate the solubility by means of the catalytic effect of an acid formed by exposure.
(3) A resist material is formed from an acid generator, crosslinking agent, and base resin. The base resin is cured (crosslinked) by the crosslinking agent in an acid-catalyst condensation reaction, and this crosslinking reaction considerably lowers the solubility in a water solution of alkali.
Thus, according to the resist materials of the chemical-sensitization type, each molecule of the catalyst (acid) contributes to a plurality of chemical reactions. Theoretically, therefore, these resist materials have a higher sensitivity to light and higher light transmission in a short-wavelength region than those of the conventional ones. If excimer laser beams are used with the resist materials of the chemical-sensitization type, therefore, the light intensity distribution with respect to the film thickness direction can be eased, so that a line width of 0.3-.mu.m level can be tackled. In forming patterns on a resist film of the chemical-sensitization type described above, however, the plane shape and depth-direction shape (contact hole shape) of each pattern are deteriorated in baking and developing processes after exposure.
In the case of the positive resist, an acid (hydrogen ions) is generated in the resist in an exposed portion 3b, as shown in FIG. 1A, and the acid reacts to alkali components to form a neutralized layer 4, as shown in FIG. 1B. Since a compound insoluble or hardly soluble in the developing agent is formed in this neutralized layer 4, an insolublilized portion 5 remains after the development, as shown in FIG. 1C. Since the insolublilized portion 5 narrows the opening of a contact hole 6, etching defects are caused in the subsequent processes. In these drawings, numerals 2 and 3a denote a silicon substrate and an unexposed portion, respectively.
The alkali components are derived from a very small amount of ammonia contained in the air or an amine generated from paints on the wall or HMDS (hexamethyldisilazane) used in adhesion treatment. It is estimated that these trace alkali components react to the acid in the vicinity of the surface of the resist film.